Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper. Set aside.
Add softened butter and brown sugar to the bowl of your stand mixer and cream together until smooth.
Add in the egg and vanilla and and mix in.
Sift the all purpose flour into a mixing bowl and add baking soda, baking powder, cornstarch and salt and mix together.
Add the dry ingredients in with the wet and mix in by hand (don't use your electric mixer for this part) until crumbly.
Then add in the milk and mix into a cookie dough.
Add the chocolate chips and mix in.
Roll into balls and place them evenly on the parchment lined baking tray.
Bake for 10-12 minutes. If you like your cookies ultra soft then bake them for 10 minutes. If you like them slightly crunchy on the outside but still soft on the inside then bake them for 12 minutes and even up to 14 minutes. We baked these cookies for 12 minutes for a slightly crunchy outside and a soft inside.
Once baked press some more chocolate chips (or chocolate chunks) into the tops of the still warm cookies.
Leave the cookies on the tray to firm up and cool before moving them.